Signal processing for digital video disk (DVD) applications, for example, conventionally requires amplifiers which can be programmed with a wide gain range, and are high performance in terms of the operating frequency. A data read channel is an example in which signal processing requires a gain-programmable amplifier.
FIG. 1 illustrates four signals A, B, C and D which have different amplitudes and are voltage-added in a node 1, and then amplified by an amplifier circuit 2 which sets the gain to obtain a sum signal. The sum signal is sent to an equalizer circuit 3 and finally to a buffer 4. The intended result is a dB linear gain variation with a linear variation of the current.
A gain setting circuit of the prior art is shown in FIG. 2, in which a differential input stage, designated by the reference numeral 10, is connected to a differential output stage, designated by the reference numeral 11. The gain A.sub.v is determined by a resistance ratio multiplied by a current ratio. The resistance ratio is determined by the ratio between the resistances of the output stage with respect to those of the input stage. The current ratio is the ratio between the current of the output stage and the current of the input stage. Accordingly, the gain A.sub.v is determined by the following relation: ##EQU1##
The above-described circuit is effective for gain programming since the gain can be changed not only by varying the resistive ratio, but also by primarily varying the current ratio. Varying the resistive ratio is difficult to implement, and in any case, requires a large circuit area. Varying the current ratio can be achieved easily with a modest use of the physical area of the device. Although the circuit of FIG. 2 is efficient for gain programmability, it is affected by drawbacks due to the high noise present at the output. The noise is amplified by the current ratio between the output stage and the input stage, and this is unacceptable whenever noise is a negative factor.